1. Field of the Invention
The present invention relates to a boost pressure control system for a vehicle engine.
2. Description of the Related Art
The boost pressure of an engine equipped with a supercharger is usually controlled in accordance with a load of the engine, such as an engine speed and a degree of opening of the throttle valve. Also, the boost pressure is controlled according to a degree of acceleration required by a vehicle driver, so that a required engine torque for the acceleration is obtained by increasing the boost pressure.
Japanese Unexamined Patent Publication No. 61-28716, for example, discloses this type of boost pressure control system, which is provided with a mechanical supercharger (i.e., a supercharger driven by the engine crankshaft) and an air bypass valve which controls an amount of air flow recirculated from a discharge side of the supercharger to an inlet side thereof.
This system utilizes a speed of opening of the throttle valve as a parameter representing a degree of acceleration requirement, and controls the operating speed of the air bypass valve according to the speed of opening of the throttle valve. Namely, when the speed of opening of the throttle valve is a large, positive value, this denotes that the degree of acceleration required is large, and the air bypass valve is closed at a higher speed determined by the speed of opening of the throttle valve, and thus the boost pressure is rapidly increased. Conversely, when the speed of opening of the throttle valve is a large, negative value, the air bypass valve is opened at a higher speed, and thus the boost pressure is rapidly lowered. This control enables the engine torque to be rapidly increased or reduced in response to the vehicle drivers need for an acceleration or deceleration.
Japanese Unexamined Patent Publication No. 60-75733 discloses a boost pressure control system in which the boost pressure is controlled in accordance with an engine speed and a degree of opening of the throttle valve. In this system, the boost pressure is increased for a predetermined time when the speed of opening of the throttle valve exceeds a predetermined speed. This control enables the engine torque to be increased during the acceleration. The boost pressure is returned to a set value for a normal operation, after said predetermined time has elapsed.
As explained above, the boost pressure control systems of the prior art control the boost pressure such that the boost pressure is increased to obtain a required acceleration torque when the degree of acceleration required becomes larger than a predetermined value.
In some cases, however, the boost pressure drop suddenly after the completion of the acceleration, in the prior art, and the resulting drop in the engine torque causes a deceleration of the vehicle larger than expected by the vehicle driver.
To compensate for this larger deceleration, the vehicle driver must readjust the amount of accelerator pedal depression, and this need for extra controls is a strain on the vehicle driver.
For example, in the boost pressure control system in the JPP, 28716 the opening of the air bypass valve is begun regardless of the engine load when the speed of opening of the throttle valve becomes a negative value at the completion of the acceleration. Nevertheless, when the engine speed drops after the completion of the acceleration, the speed of the mechanical supercharger drops in accordance with the engine speed, and therefore, even if the degree of opening of the bypass valve is kept at a constant value, the boost pressure of the engine drops with the drop in the engine speed. Consequently, if the opening of the bypass valve and the drop of the engine speed occur at the same time, the boost pressure drops more rapidly and the deceleration of the vehicle becomes greater than expected by the driver.
Also, in the boost pressure control system of JPP, 75733, the increased boost pressure is returned to a normal set value when a predetermined time has elapsed even if the acceleration is continued, and this causes a sudden drop in boost pressure and a resulting unexpected drop in the engine torque.
An object of the present invention is to provide a boost pressure control system by which the boost pressure can be rapidly increased upon an acceleration, and can avoid a sudden drop of the boost pressure at the completion of the acceleration, whereby a sudden drop of the engine torque is also avoided.